1. Field of the Invention
The present invention relates generally to a hard disk drive and, more particularly, to a method and apparatus for performing adaptive feedforward control during track seek operations with the disk drive. Even more particularly, the present invention is directed toward such a method and apparatus that perform adaptive feed forward control with reduced noise generation.
2. Description of the Related Art
The present invention uses an adaptive feedforward device to provide improved servo control of a disk drive head actuator during seek operations. It applies adaptive feedforward control during the deceleration phase of a seek maneuver to reduce the current gain needed to carry out the maneuver, thereby improving system performance by reducing current control errors.
It has been found, however, that application of feedforward current control signals specifically to the deceleration phase of a seek operation tends to induce a certain level of noise in the servo control signals generated by the servo control apparatus. This noise occurs as the apparatus shifts from acceleration to deceleration and appears as a rapid variation or oscillation in the current control commands of the servo apparatus at the shift point.
General servo control technology has followed parallel and occasionally intersecting paths for improvements in feedforward control and noise compensation, respectively. U.S. Pat. No. 3,940,594, for example, shows a process controller that eliminates mode-switching noise by the rather direct expedient of freezing the actuator command signal and then gradually mixing the target state signal through a settle circuit. Alternatively, U.S. Pat. No. 3,961,234 shows a system for industrial process control using electrical actuators that adaptively filters measurement noise in a high-gain feedback loop. Here the system avoids dithering due to noise by limiting the time allowed for control signals when the error measurement signal is small.
The disk drive servo control arts, in particular, have also confronted the general problem of noise arising in control systems. For example, U.S. Pat. No. 4,642,541 discloses a servo system that uses a filter network in track following mode to generate a substantially noise-free position error signal. The significance of this improvement does not extend far beyond its direct context, however, because it does not consider other noise processes such as mode-transition noise. This patent also does not consider the use of adaptive control, noise reduction in seek operations, or noise arising in control signals themselves.
U.S. Pat. No. 4,893,068 for a Digital Employing Switch Mode Lead/Lag Integrator to D. D. Evans, shows a sophisticated digital servo control system employing an integrator located outside the conventional feedback control loop. This configuration permits the integrator to be applied to both velocity error signals in seek-type operations and position error signals in position holding operations. The system mitigates perturbations arising from switching between velocity and position error integrator inputs (i.e., mode-switching noise) by mixing the output from the previous integrator cycle with the output from the current integrator cycle. Unfortunately, the rather specialized structure employed in the disclosed system does not easily lend itself to application in other servo control contexts. It is not clear how one would adapt the system of this patent to mitigate current oscillations arising from an adaptive control feedforward network while not effectiveness of the feedforward signal.
The disk drive control arts have explored the uses of adaptive control in some contexts, including to minimize transients arising from shifts between track seeking and track following modes. U.S. Pat. No. 4,697,127 shows an adaptive control technique that estimates the forward gain of an open loop system to compensate for variations in plant parameters. This system minimizes seek-follow transition transients by applying, during seeking, a velocity command profile selected to match the normal modes (i.e., eigenvectors) of the system in track following mode. Prospective selection of seek command profiles enables the system to blend the end of the seek operation with the beginning of the track following operation, thus preventing the generation of transient components in the system response.
The sophisticated approach taken in the '127 patent also unfortunately limits the extent to which the disclosed system can be adapted to other contexts. First, its use of forward gain estimation and accordant selection of seek command profiles necessarily depends upon a sophisticated plant model and substantial computation to achieve its desired results. The disclosed system also considers only process noise (such as might arise from resonances, unmodeled bearing drag, and so forth) and measurement noise. In particular, it does not consider the constellation of problems arising from noise induced in the control signal itself.
U.S. Pat. No. 5,128,812 provides a disk drive servo control system that eliminates high frequency components from the velocity error signal to be amplified in a feedback loop. The previously existing configuration included a low pass filter to remove undesired high frequency components of the velocity difference signal in a feedback control loop. The improvement of the patent consists in a filter with variable cutoff frequency to minimize the phase lag the filter introduces into the error signal while maintaining stability of the system.
The system of the '812 patent provides an important improvement directed toward a specific problem in servo control, but it also carries several limitations. The careful balance it observes, between reducing phase lag and preserving system stability, rather clearly acknowledges the considerable complications that are known to arise from employing low pass filter networks in servo control loops. Simple filtering appears, from this patent, to have been abandoned by the art, at least for processing servo control signals. This patent also does not consider the mitigation of transient components in the context of feedforward loops or through digital signal processing techniques.
The known approaches to eliminating noise from signals in servo control systems thus have not offered an obvious solution to the transient noise problem I have noticed in using adaptive feedforward control to reduce current control errors. Such a solution should be easily and economically implemented in the adaptive feedforward device of my previous invention and should effectively eliminate the control signal noise that arises there. It should not hinder system stability and also should not reduce the effectiveness of the control system in the way that low pass filters reduce the effectiveness of velocity feedback systems by introducing phase lags. Desirably, this noise control solution should be as simple as possible so that my adaptive feedforward control system employing this solution will be applicable in as many differing contexts as possible.